This invention relates to a method and system for implementing loop diagnostics in a polling loop security system.
Security systems that comprise a number of devices, or modules, interconnected to a control panel by a communications bus, are well known in the art. Security modules typically are used to monitor an area of space or a specific access point, and report to the control panel if there is a change in status. For example, modules exist that monitor opening of doors or windows, that determine if an intruder has entered the premises such as by passive infrared surveillance techniques, or that determine if a fire has started, etc.
The modules used to monitor areas often utilize sealed or unsealed contacts which are either mechanically, magnetically, or electrically operated. These contacts are connected via electrical wire at distant positions from the control panel, which may be over thousands of feet in length. Both the resistance of the wire connections, including the resistance of the contacts themselves, can increase or decrease in time due to temperature, humidity, and general aging conditions.
A given contact is sometimes terminated with a fixed resistor which has a value that will allow the control to determine one of three states of the protected loop; normal, shorted, or opened. In this case, the shorted and opened conditions are abnormal conditions of alarm or trouble. Other contacts are monitored simply for its open or closed state. In either case, it is known that these protective loops can deteriorate with time and it would be of great advantage to the security system if the level of deterioration can be determined prior to that loop causing either a false alarm or false trouble condition.
The purpose of this invention is therefore to provide a diagnostic means of quantitatively determining the level of deterioration of the protective loops as employed in the subject polling loop security system utilizing diagnostic circuits employed in the system modules.
The present invention is therefore a security system comprising a control panel and a plurality of individually-addressable security system modules connected to the control panel on a loop data bus. Each of the security system modules has a reference generating means for generating a variable reference voltage signal, which is controlled by control data received from the control panel. Each module also has means for selecting a loop input signal for analysis from a plurality of available loop input signals, and comparing means for comparing the selected loop input signal to the reference voltage signal. The comparing means generates an output signal when the selected loop input signal exceeds the reference voltage signal. The modules also have means for indicating to the control panel via the loop data bus the state of the reference generating means when the comparing means generates an output signal, whereby the control panel can determine the value of the selected loop input signal.
The reference generating means includes a counter that has a clock input for incrementing the counter and a plurality of output bits. The clock input is configured to receive the control data from said control panel. There is also a digital-to-analog converter that has a plurality of input bits and an analog output for generating the variable reference voltage signal, wherein the input bits are coupled to the output bits of the counter. As a result, when the control data is a bit stream input to the counter, the counter will generate a reference voltage signal that increases as the number of clock pulses input to said counter increases.
The means for selecting a loop input signal for analysis from a plurality of available loop input signals includes a plurality of transistors, each of the transistors configured to switch a corresponding one of the loop input signals in accordance with a multiple-bit input selection command word received from said control panel.
The control panel transmits a serial control word including the input selection command word and the control data, wherein the control data is a plurality of serial clock pulses.
The means for indicating to the control panel via the loop data bus the state of the reference generating means when the comparing means generates an output signal includes means for suppressing the signal line comprising the serial clock pulses, whereby the control panel is able to determine the counter value at which the comparing means generates an output signal by analyzing the number of serial clock pulses that were sent to the module before the signal line comprising the serial clock pulses is suppressed.
The counter value may be used by the control panel to determine the relative voltage of the selected input with respect to a loop voltage applied to the module. The counter value may alternatively be used by the control panel to determine the actual voltage applied to the module by selecting a known reference voltage for measurement by the control.